1. Field of the Invention
The present invention pertains to ferroelectric channel waveguides for use in providing optical sum-frequency generation and to at least one method for fabricating efficient ferroelectric channel waveguides for such use.
2. Description of the Prior Art
It is well-known that certain classes of optical crystals have dielectric properties responsible for nonlinear responses in the crystals in the presence of appropriate optical fields. The nonlinear responses can manifest themselves as exchanges of energy between fields of different frequency. Of this type, two of the more important nonlinear responses are second harmonic generation in which part of the energy of an optical wave propagating in a crystal is converted to a wave of twice the frequency of the original and sum-frequency generation in which a strong pump at one frequency can produce radiation at two other frequencies whose sum equals that of the pump frequency or in which two or more pumps of different frequency will produce, in addition to their individual second harmonics, optical frequencies equal to the sum of the two.
Recently, much activity has taken place in the field of second harmonic generation in LiNbO.sub.3 channel waveguides. One reason for this activity is the desire to produce a miniaturized light source of coherent radiation at small wavelengths, for example, to produce a coherent source of blue light. Specifically, an abstract entitled "Miniaturized Light Source Of Coherent Blue Radiation" by T. Taniuchi and K. Yamamoto, published as abstract WP6 in the conference proceedings of the Conference on Lasers and Electro Optics (CLEO) sponsored by the Optical Society of America, Washington, D.C., 1987, discloses the frequency doubling of 0.84 um laser radiation in a proton-exchanged LiNbO.sub.3 waveguide at room temperature. The abstract discloses the following features of the apparatus: (1) frequency doubling of the laser diode radiation is achieved by using a Cerenkov radiation scheme in the proton-exchanged LiNbO.sub.3 waveguide; (2) the proton-exchanged LiNbO.sub.3 waveguide is advantageous because it has a high resistance to optical damage; and (3) the Cerenkov radiation scheme automatically provides phase matching between a guided mode of the fundamental wave and a radiation mode of the harmonic wave because the spectrum of the radiation mode is continuous. The disclosed waveguide was fabricated by proton-exchange using pyrophosphoric acid (H.sub.4 P.sub.2 O.sub.7) as a proton source. At room temperature, a spinner was used to coat the acid on the surface of a z-cut LiNbO.sub.3 substrate having a tantalum mask, and the substrate was then heated at 230.degree. C. As a result, the extraordinary refractive index was raised by about 0.145 at a wavelength of 0.63 um add had a step-index profile. The waveguide was formed for propagation in the y-direction on the substrate and was 2 micrometers wide, 0.47 micrometers deep, and 6 mm long. The abstract discloses that the output from a high-power GaAlAs laser diode having a wavelength output at 0.84 um and an output power maximum at 120 mW passed through a half-wave plate and was then coupled into one end of the waveguide by a focusing lens having a numerical aperture (NA) equal to 0.6. The half-wave plate changed the polarization direction of the laser output to provide coupling to a TM-guided mode. The abstract further discloses that a maximum output power of 1.05 mW at 0.42 micrometers was achieved for a fundamental pump power of 120 mW and that the total power coupled into the waveguide was 65 mW.
Further, an abstract entitled "Second Harmonic Generation By Cherenkov Radiation In Proton-Exchanged LiNbO.sub.3 Optical Waveguide" by T. Taniuchi, K. Yamamoto, and Y. Fujii, published as abstract WR3 in the conference proceedings of the Conference on Lasers and Electro Optics (CLEO) sponsored by the Optical Society of America, Washington, D.C., 1986, discloses second harmonic generation by Cerenkov radiation in a proton-exchanged LiNbO.sub.3 waveguide by automatic phase matching between a fundamental guide mode and the continuous spectrum of the radiation mode of a second harmonic wave. The abstract discloses coupling 1.06 micrometer radiation output from a cw Nd:YAG laser into one end of the waveguide via a 40X microscope objective. Further, the abstract discloses that the output power of the second harmonic is proportional to the square of the pump power which was coupled into the waveguide and that a maximum conversion efficiency from pump radiation to second harmonic radiation was achieved when the waveguide width was 2.5 micrometers, the waveguide thickness was 0.55 micrometers and the waveguide length was 6 mm. Still further, the abstract notes that the apparatus could serve as a practical coherent light source for laser printer applications in the green and blue wavelength region.
Lastly, an abstract entitled "New Proton-Exchange Technique For LiNbO.sub.3 WaveGuide Fabrication" by K. Yamamoto and T. Taniuchi, published as abstract TUH2 in the conference proceedings of the 6th International Conference on Integrated Optics and Optical Fiber Communication (OFC/IOOC) '87, sponsored by the Optical Society of America, Washington, D.C., 1987, discloses a method for fabricating a proton-exchanged waveguide in LiNbO.sub.3 by using pyrophosphoric acid as a proton source. The abstract discloses that pyrophosphoric acid was chosen as a proton source primarily because it is a liquid at room temperature and because it has a high coefficient of viscosity. The high coefficient of viscosity allows pyrophoshoric acid to be coated on the surface of a LiNbO.sub.3 substrate. Further, because of its high proton concentration compared to that of benzoic acid, it can be used to form a high-index waveguide. Stripes were fabricated using a conventional photolithographic technique. In particular, a tantalum (Ta) mask was used because pyrophosphoric acid attacked other metals. The Ta mask was patterned by CF.sub.4 reactive ion etching; pyrophosphoric acid was then coated on the z face of LiNbO.sub.3 substrates using a spinner at 300 rpm for 40 seconds; and the substrates were then baked in a pyrex tube at temperatures ranging from 150.degree.-280.degree. with diffusion times varying from 5 minutes to 6 hours. After the exchange process, the substrates were washed in ionized water to remove excess pyrophosphoric acid.
As one will readily appreciate from the activity discussed above, a coherent blue and/or green light source is desired and, in particular, for use, in laser printer applications. Further, proton-exchanged LiNbO.sub.3 channel waveguides appear to provide a potential structure for use in producing such sources of radiation. However, the efficiencies that have been achieved in the prior art are not sufficient to provide a practical device which can efficiently operate in a commercial environment.
Consequently, it is a primary object of the present invention to advantageously solve the above-described problems in the prior art by providing a high-efficiency source of optical sum-frequency radiation in ferroelectric channel waveguides and, in particular, for producing second harmonic radiation therefrom.